Antibodies are an integral component of the immune response to infection. They neutralize infectivity, mediate lysis of infectious agents, augment clearance of microorganisms by neutrophils and macrophages, and participate in antibody-dependent cell-mediated cytotoxicity. However, the effectiveness of these effector functions depends on how well antibody recognizes the antigen. Relative to the primary response, the secondary antibody response is characteristically rapid, of greater magnitude and of higher affinity toward antigen. The higher affinity exhibited by these antibodies is explained by the finding that the antigen binding domains of antibodies expressed by responding B lymphocytes change as a result of mutations which accumulate in the variable (V) gene segments of immunoglobulin (Ig) genes. Studies on the molecular mechanism underlying this process, somatic hypermutation, have been mostly limited to in vivo models. This is due to the unavailability of suitable B-cell lines in which somatic mutation is active, and the apparent requirement for T-cell help and a specific microenvironment in which this B-cell activity may be regulated: the germinal center. 'ne nature of this regulation remains uncharacterized. Therefore, the primary objective of this proposal is to characterize the regulatory events involved in the control of somatic hypermutation with the goal of reconstituting the mutation process in vitro. The specific aims of the proposed research are: 1) To characterize the minimal requirements for reconstitution of somatic mutation activity in SCID mice. The goal is to dissect the B- and T-cell interactions which are necessary and sufficient to generate B cells with mutated V regions in vivo; 2) To determine whether the B-cell populations thought to be mutating in vivo do so in vitro. Can B-cell growth and differentiation factors induce or support this activity in defined B cell subsets, when proliferating in response to polyclonal activators? 3) To examine T- and B-cell contact requirements, and the role of accessory cells, such as follicular dendritic cells in the induction and maintenance of somatic mutation activity.